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My 75 has 11 fancy goldfish, 4 axolotls, and a stable population of wild type (feeder) guppies, but they will not all be staying in there. I would keep a maximum of six in a 75. Their body alone (not counting the fins) can be the size of a man's fist (about softball size).

I do not think bottom feeders are needed if you have enough filtration/flow and sand, which would be my recommendation. Not just because in general I have found sand to be the best substrate in general, but in this case also because some goldfish have died because they swallowed gravel and it blocked everything up.

I do not have any bottom feeders in my tank and it stays clean, I do not even need to vacuum. Goldfish themselves are really a bottom feeder. They love to sift through sand in search of food.